Fluid operated rock drill having an independent rotation motor

ABSTRACT

A rock drill having a pneumatically reciprocable piston hammer to pound a striking bar. Live air is caused to be applied continuously to one end of the hammer and alternately to the opposite end. A fluid motor is operable to reciprocate a drive piston relative to live air feed ports and to the hammer to cause development of differential pneumatic pressures alternately at opposite ends of the hammer to reciprocate the latter. Porting of live air to opposite ends of the hammer is arranged so that when the striking bar is withdrawn from or breaks through the work, the hammer will automatically stall even though the drive piston continues to reciprocate. Except during moments of impact of the hammer with the striking bar, pressure air is caused to continuously blow through a hole in the bar to clear away debris at the work end of the tool. A second fluid motor operates independently of the rock drill motor to transmit rotation through a gear train coupling to the striking bar as the latter is being pounded by the hammer. The coupling absorbs undesirable shocks arising between the striking bar and the second motor.

United States Patent Amtsberg et al. 1 51 Aug. 22, 1972 FLUID OPERATED ROCK DRILL Primary Examiner-Emest R. Purser HAVING AN INDEPENDENT Attorney-Stephen J. Rudy ROTATION MOTOR [72] Inventors: Lester A. Amtsberg; Paul J. [57] CT Bilodeau, both f Utica; s b fl n A rock dr1ll having a pneumat1cally reclprocable J dd white b n f piston hammer to pound astriking bar. Live an is caused to be applied continuously to one end of the [73] Asslgnee: Chlcago Pneumatc Tool C0mPany, hammer and alternately to the opposite end; A fluid New York motor is operable to reciprocate a drive piston relative [22] Filed; No 970 to live air feed ports and 'to the hammer to cause development of differential pneumatic pressures all l PP N04 86,403 temately at opposite ends of the hammer to reciprocate the latter. Porting of live air to opposite ends of the hammer is arranged so that when the strikliil lifjilii:11:1:iiiiiiiiiiiiiiiliillz13320533932 mgbariswithdmwnfmmprbreaksthroughmework. 58 Field of Search ..173/14 75 76 105 116 the hammer will ammaflcally hwgh the drive piston continues to reciprocate. Except during [56] References Cited moments of impact of the hammer with the striking bar, pressure air is caused to continuously blow UNITED STATES PATENTS through a hole in the bar to clear away debris at the work end of the tool. A second fluid motor operates 215361971 1/ 1951 wyandt et 7 independently of the rock drill motor to transmit rota- 1,954,411 et a] tionthrough a g train p i g to the Striking bar 1,340,505 5/1920 Srnlth ..l73/ 105 X as the latter is being pounded by the hammen The 3,570,608 3/1971 Erma ..l73/ll6 coupling absorbs undesirable shocks arising between the striking bar and the second motor.

8 Claims, 1 1 Drawing Figures Patented Aug. 22, 1972 3 Sheets-Sheet 1 ap 3 wt $1 E 7' 7% m .m I VA/ 6Q v u Maw R \N S w k \s Q \QRQ Q NM NM w \m, a 3 9k Patented Aug. 22, 1972 3 Sheets-Sheet 2 TOR5 wr /r5354; 5/4005/10 ,1/ F! T000 F G my? PA (It J: BYJEBJI 7' lr? Stir (A4.

ATTORNEY Patented Aug. 22, 1972 3,685,593

3 Sheets-Sheet 5 /02 v FIG. 7 8 w W -35 ATTORNEY FLUID OPERATED ROCK DRILL HAVING AN INDEPENDENT ROTATION MOTOR BACKGROUND OF INVENTION The invention is concerned with a percussive rock drill operable by the development of differential pneumatic pressures alternately at opposite ends of a piston hammer, and having an independent motor to rotate its striking bar as the latter is being pounded by the hammer.

A rock drill of a differential pressure type is known from US. Pat. No. 2,609,813. The mode of operation of this known tool is materially diflerent from that of the present invention in that it makes use of ambient atmospheric pressure in its operations, whereas the subject invention uses live air. The result is an improvement in the operating efficiency of the subject tool over that of the known tool. Further, the manner of porting operating air to the chamber of the piston hammer of the present invention is such that the hammer will be stalled in its operation when the tool is withdrawn from or breaks through the work.

A rock drill of an independent rotation type is known from U.S. Pat. No. 3,368,634. This known tool is primarily directed to means for absorbing axial thrusts imparted to the independent rotation motor and the as sociated housing; whereas the arrangement of the subject invention is directed to absorbing various directional shocks developing in the drive between the independent rotation motor and the striking bar.

In accordance with the invention, there is provided a rock drill including a work engaging striking bar, a piston hammer reciprocable to pound the bar against the work, means for alternately pneumatically pressurizing opposite ends of the hammer to reciprocate the latter, a gear train drive connected with the bar operable to rotate the bar while the hammer is recprocating, a rotary motor operating independently of the reciprocating hammer having an output shaft for driving the gear train drive, and means in the gear train drive to absorb shocks that might develop in the drive between the striking bar and the motor.

BRIEF DESCRIPTION OF DRAWING In the accompanying drawing:

FIG. 1 is a view in longitudinal section of a rock drill illustrating the invention;

FIG. 1a is an enlarged detail of a portion of FIG. 1, directed to the ports in the piston cylinder;

FIG. 1b is an enlarged detail of a portion of the elastomeric coupling, illustrating the arrangement of an insert in the elastomer disc element;

FIG. 2 is a detail of the forward end of the intermediate housing section shown in FIG. 3;

FIG. 3 is a section on line 33-of FIG. 2;

FIG. 4 is a section on line 4-4 of FIG. 1;

' FIG. 5 is a detail of the front end of the elastomeric coupling;

FIG. 6 is a detail of the rear end of the coupling;

FIG. 7 to 9 represent a modified drive arrangement connecting an independent rotation motor with the striking bar in which:

FIG. 7 is a longitudinal sectional view;

FIG. 8 is a section on line 8-8 of FIG. 7; and

FIG. 9 is a section on a horizontal plane through the right portion of FIG. 7.

2 DESCRIPTION OF PREFERRED EMBODIMENT In the drawing is disclosed a rock drill 10 having an elongated housing 11 defined by a plurality of sections secured to one another in end-to-end relation. The housing is provided with the usual mounting means (not shown) whereby the rock drill may be supported upon a guide channel and fed in conventional manner relative to the work.

Mounted externally of the housing is a fluid driven rotary motor 29 which transmits its rotation through a gear train 31 to a chuck driver 13 disposed in a front end section of the housing. The chuck driver is coupled by means of the usual end lugs 14 to drive a chuck l5. Rotation of the chuck is transmitted to a striking bar 16 by means of longitudinal splines 17 of the chuck engaged with relatively shorter longitudinal splines 18 of the striking bar. The shank of the striking bar projects through an axial opening in a cap 20 threaded over the front end of the housing. A stop ring 21 positioned between the cap and the chuck is cooperable with forward ends of the splines 18 to limit the extent of for.- ward sliding movement of the striking bar relative to the housing. An end 23 of a bushing 24 fitted in the chuck driver is cooperable with the rear ends of the splines 18 to limit the extent of rearward movement of the striking bar as occurs when the latter is pressed against the work.

When the striking bar is pressed against the work and retracted, as in FIG. 1, to its rearward position, its striking end 26 projects into a lower expansible chamber A wherein it will be subject to impacting action by a reciprocable hammer piston 28. A fluid driven rotary motor 32 mounted to a side of the housing is operable to reciprocate a piston driver 44 relative to the hammer to effect, in a manner as will be described, reciprocation of the hammer. The motors 29 and 32 are operable independently of one another.

A stationary sleeve 39 lining the interior wall of an intermediate section 27 of the housing serves as a piston cylinder in which the hammer and the piston driver are both reciprocable.

The hammer 28 has a rear body portion 62 of enlarged diameter which has a bearing sliding relation to the surrounding wall of the piston cylinder 39. A stem portion 67 of reduced diameter depending axially from the body of the hammer is aligned with striking bar. When the striking bar is retracted and the hammer is reciprocated, a flat end 68 of the stem is adapted to pound the bar. The hammer is formed with a deep, axially extending recess 63 in its rear which has a coned side wall 64 tapering slightly downward to a substantially flat bottom 65.

The piston driver 44 has a peripheral land 42 about its rear which bears slidably upon the surrounding wall of the piston cylinder. Forwardly of land 42 the body of the piston driver is of reduced diameter. It complements, in its outer surface, the wall 64 of the hammer recess 63 and is axially receivable in the latter.

The expansible area in the piston cylinder between the hammer and the piston driver defines a pocket or compression chamber C. Air received in chamber C through the side ports 59 and 73 of the piston cylinder is subject to being compressed during relative reciprocation of the hammer and piston driver. The hammer is caused to reciprocate accordingly as differential pressures are developed at opposite ends thereof, that is, in chambers A and C.

3 Reciprocation of the piston driver is obtained by operation of the motor 32. The motors drive shaft 33 has a driving connection with a crankshaft 34 supported in cylindrical bearing blocks 35 mounted in opposed circular openings 36 of the housing. A connecting rod 37 connects the crankshaft with a wrist pin 38 extending transversely of the land 42 of the piston driver. A deep recess 46 in the rear of the piston driver accommodates the free end 47 of the connecting rod.

Recess, 46 also serves as an extension of a rear volume or expansible inlet chamber B with which a pressure air supply inlet 57 connects. The inlet is connectable with an extemal' source of live air of constant pressure. Rotation of motor 32 is transmitted through the crank elements 34, 37 and 38 to reciprocate the piston driver relative to the hammer.

Differential pneumatic pressures are caused to develop in chambers A and C as the piston drive is reciprocated. This results in reciprocation of the hammer. For this purpose, there is provided in the wall of the housing a pair of longitudinally extending main air feed passages 55 (FIGS; 34) which connect at one end through ports 56 with the inlet chamber B; and

position upon the stop ring 21, its striking end 26 is out of striking range of the hammer, and a blow hole 69 extending axially through the bar is in communication with chamber A. The hammer is limited in the extent of its forward movement by means of a coned shoulder or seat 72 of the housing with which a complementary coned shoulder 71 of the hammer is cooperable. In this forward position of the hammer, both rings of ports 59 and 73 are uncovered and connect the live air distribution annulus 53 with chamber C.

When the striking bar is retracted to itsrear position, it engages the stem 67 of the hammer and raises the hammer clear of seat 72. In this raised condition of the hammer, ports 73 are closed over by the tail end 74 of the body of the hammer but ports 59 remain uncovered, as appears in FIG. 1.

In FIG. 1 the driving motor 32 has been stopped at a stage of operation in which the crankshaft 34 has obtained the bottom of its work stroke. The hammer 28 is at this time seated over the end of the fully retracted striking bar 16 blocking the blow hole 69 to entry of air from chamber A; and chamber C is in communication through ports 59 with the air feed annulus 53. To resume operation of the tool, live air is fed through the inlet 57 into chamber B. It then passes through ports 56 into passages 55 from where itflows in part through ports 54 to the distribution annulus 53 and in part through ports 56a to chamber A. The pressure of aid in chamber A exerts a rearwardly acting force upon the hammer but pressure air from the distribution annulus passing through the presently uncovered ports 59 to chamber C provides a forwardly acting force upon the hammer counterbalancing that in chamber A. To effect reciprocation of the hammer relative to the striking bar, it is required'to develop difierential pressures al- I a relaxed pressure condition to develop in chamber Cv relative to the opposing pressure acting on the hammer in chamber A. The consequent pressure differential developing in chamber A forces the hammer rearwardly. on a return stroke away from the striking bar to close over ports59 and block further feeding of air from the annulus 53 to chamber C. Some of the air entering chamber A from ports 56aescapes through the blow hole 69 to the .work bore as the hammer moves away from the striking bar. This blow air serves to clear debris from the work. Despite the air loss through the blow hole, the pressure in chamber A remains adequate to continue forcing the hammer rearwardly. This is because flow through the blow hole is relatively restricted and its loss is compensated for by the larger volume flow through the larger ports 560 from the main feed passages55 to chamber A. Further, the velocity of striking bar 16 is restored to a hard work surface and air entering chamber A at this time is caused to increase as air is forced back from chamber B through ports 56 into passages 55 by the rearwardly moving piston driver. The resultant differential in pressures on the front and back ends of the hammer causes the hammer to follow the piston driver rearwardly. Until shortly after the piston driver has reached the top of its return stroke and starts downward, the hammer continues to move telescopically upwardly relative tothe If, while the tool is in operation, the striking bar should be withdrawn from the bore, or if it should suddenly break through the work into a vacant area, it will, as a consequence, be dropped or driven forwardly to limit in its lowermost position on the stop ring 21; and the hammer will follow the bar in this movement sufliciently to carry its tail end 74 forwardly and clear of the ports 73. The coned seat 72 of the housing will cooperate with the coned shoulder 71 of the hammer to limit the forward movement of the hammer. As a result, the hammer will become automatically stalled in its dropped condition as air from annulus 53 passes through both rings of ports 59 and 73 to chamber C to counterbalance the pressure of air in chamber A. The hammer will remain stalled despite continued operation of the motor 32 and reciprocation of the piston driver because of a continued greater volume flow through ports 59 and 73 to chamber C than occurs to chamber A. The hammer will remain stalled until the retracted to re-elevate the hammer to its normal operating condition, as in FIG. 1.

This stalling action of the hammer following withdrawal or break-through of the striking bar is of considerable advantage in that it avoids repeated idle reciprocation of the hammer and consequent undesirable pounding against the housing seat 72, as might otherwise occur.

A decided advantage of the cup-like forms of thepiston hammer and the piston driver is that it allows them to move telescopically relative to one another. This results in considerably lesser weight of these elements and consequent better operating efficiency than would otherwise be the case were these elements substantially solid. Further, this telescoping feature permits use of a relatively shorter piston cylinder than would otherwise be required. The light weight of these components, because of the deep recesses in their rear ends, also result in reduced overall weight of the tool.

During the time the hammer is being reciprocated to pound the striking bar 16, the chuck driving motor 29 is independently operating to rotate the striking bar through the gear train 31. The gear train is arranged in an offset section 75 of the front end of the rock drill housing. The gear train includes a main driving gear 76 journaled in bearings 77. An idler gear 78 supported in bearing 79 connects the driving gear with a surface gear 81 of the chuck driver 13. The motor 29 is bolted by means of an end flange 82 to the gear train housing section 75 in such manner that the axis of its drive shaft 83 extends parallel to the axis of the rock drill housing. The motor drive shaft 83 is connected by means of an elastomeric flexible coupling 84 to the main driving gear 76.

The coupling 84 (FIGS. 1, 1b, 5, 6) functions to absorb torque strain and undesirable backlash that might at times develop between the gear train 31 and the motor drive 83. The coupling includes a thick disc 85 of elastomeric material such as Neoprene or other resilient material such as rubber. The disc has a central axial opening 86. Formed in the disc concentrically about the opening 86 is a ring of holes 87 spaced equally apart, here six in number. A separate openended tubular metal insert 88, preferably of light weight material such as aluminum alloy metal, is bonded in each of the holes 87 of the disc. Every other insert projects slightly from one face of the disc and the alternate inserts similarly project from the opposite face, as in the detail FIG. 1b. The hub 91 of an adapter 92 (FIGS. 1, 6) is internally splined upon the drive shaft 83 of the motor and extends axially with a surrounding clearance into the disc opening 86. Three coplanar flanges 93, extending radially from the hub 91 of the adapter 92 and spaced equally apart overlie the rear face of the disc 85. Each flange has a bolt receiving aperture registered with an axial hole of a separate insert 88. Each flange abuts the projecting end of its related insert and is fastened to it by means of a bolt and nut element 94. A second adapter 95 (FIGS. 1, 5) has an axially extending stub shaft 96 splined into the main gear 76 in axial alignment with the drive shaft 83. Three equally spaced co-planar flanges 97 extending radially from the rear of the stub shaft 96 overlie the forward face of the elastomeric disc 85; and each flange has a bolt receiving aperture registering with a sorbing undesirable torque reactions.

separate hole of a separate insert 88. Each flange abuts the projecting end of its related insert and is fastened to it by means of a bolt and nut element 98. The disc is of adequate axial thickness so that the hub 91 of the adapter 92 does not extend through the central opening 86 to contact the second adapter 95.

The coupling 84 is of decided advantage. It avoids metal-to-metal contact of the motor drive shaft 83 and the gear train 31. The torsional characteristics of the elastomeric disc 85 permits it to absorb shock and backlash and thus avoids undesirable torque reactions to the gear train 31 and motor 29; such as might develop under load during initial transmission of rotation to the gear train during starting or when the striking bar 16 becomes tight in its bore and tends to resist rotation. The slight axial projection of the inserts 88 beyond the faces of the disc 85, as best seen in FIG. 1b, serves to maintain a slight clearance and a non-compressive relation of the adapters 92 and to the corresponding faces of the disc, thus enabling the full torsional characteristics of the disc to be utilized in ab- In FIGS. 7 to 9 is discloses a modified arrangement for transmitting independent rotation to the striking bar. This arrangement is designed to absorb variously directed thrusts and shocks transmitted back and forth between an independent rotation motor 99 and the striking bar 16. It includes an offset housing portion 100 extending transversely of the front head 101 of the rock drill. The offset portion has an end wall 102 to which the housing of the motor 99 is bolted. The-motor is operable independently of the rock drill motor 32. The motor 99 is of an air driven radially slidable vane type, here of the roller vane type. However, a hydraulically powered motor may also be used.

The motor 99 has a hollow rotor shaft, the ends 104 and 105 of which are respectively supported in bearings 106 and 107. Axially aligned with the rotor shaft is a hollow worm gear shaft 108 supported at its ends in bearings 109, 110.

Received axially within the rotor and worm gear shafts is a torsion bar 117. The outer rotor shaft end 105 has an internal splined driving connection at 118 with one end of the torsion bar. The opposite end of thebar has a splined driving connection at 121 with the worm gear shaft 108. The portion of the torsion bar between its splined ends is of relatively reduced diameter so asto provide desirable torsion characteristics for the bar.

The worm gear shaft 108 has a straight splined driving connection 125 with a worm gear 126 which in turn drivingly engages a surface gear 127 of the chuck driver 13 of the rock drill. It can be seen that rotation of motor 99 will be transmitted through the train of driving connections to the striking bar 16 of the rock drill. The torsion bar 117 serves to absorb shocks, particularly those resulting from overloading torque developing in the drive between the striking bar 16 and the motor 99.

The worm gear 126 is arranged so that shocks developing between the motor and the striking bar will be absorbed, particularly those of an axially directed nature, such as might develop during pounding of the striking bar by the hammer. Its straight splined engagement at 125 with the worm gear shaft permits the worm gear to slide axially to a limited degree relative to its shaft when subjected to axially directed shocks or thrusts. A pair of shock'absorbing units 129, one of which is located adjacent each end of the worm gear, serves to absorb these axially directed shocks. Each shock absorbing unit is slidably received upon the worm gear shaft. It includes inner and outer thrust bearing rings 130, 131 bonded to an intermediate elastomer ring 132. One of the shock absorbing units is disposed with its inner ring in abutment with one end face of the worm gear and with its outer ring in abutment with the inner race of bearing l09. The other unit is similarly disposed between the opposite end face of the worm gear and the inner race of bearing 110.

It can be seen by means of this modified arrangement that shocks being transmitted back and forth between the motor and the striking bar through the train of drive connections will be absorbed by the torsion bar and the presses upon a shock 'or force being axially applied-to the related inner thrust ring; and is adapted toresiliently expand to normal condition upon the force being relieved. It is apparent that as various thrusts imparted to the worm gear force it slidably in one direction or the other upon the worm gear shaft, the undesirable eflects of such thrusts will be absorbed or dampened by one or the other of the shock absorbing units 129.

Themodified structure shown in FIGS. 7-9 is also advantageously designed so as to utilize the oil mist inherent in the operating air to lubricate its various components. In this respect, operating air carrying oil mist and entering-the inlet 133 to the motor chamber 134 exhausts. in part through ports (not shown) of the motor housing and also flows through connecting ports to the bearings 107 andv 106. Air passing through bearing 107 at the right end of the rotor shaft 105 enters the center hole 135 in the torsion bar and flows out the opposite end to bearing 110 through which it flows to lubricate the worm gear from the left. Air passing through bearings 106 and 109 flows to lubricate the worm gear from the right. The worm gear, in turn, carries the lubricant to the surface gear 127 of the chuck driver 13. i

What is claimed is:

l. A rock drill including a casing having a piston chamber provided with-a shoulder at its lower end, a floating piston hammer reciprocable in the chamber relative to the shoulder, the hammer being of cup-form provided with an axially projecting stem, a work steel having a retracted position in the casing wherein its top I end is subject to being pounded by the stem as the hammer reciprocates and wherein the hammer is disabled by the work steel from engaging the shoulder, a pressure air inlet having passages for feeding pressure air to opposite ends of the piston hammer, and cupform means reciprocablein and out of the cupof the I I hammer for developing pneumatic pressure differentials of such air alternately at opposite ends of the hammer to reciprocate it during the time the work steel is in its retracted position, wherein the work steelhas a surface gear intermediately of its ends, a rotary motor operating independently of the reciprocating hammer has an output shaft, and gear train means drivingly couples the output shaft of the motor with the gear of the work steel; characterized in that the gear train means includes a worm gear shaft carrying a worm gear having a driving connection with the gear of the striking bar, and a torsion bar drivingly connecting the output sh of the motor with the worm gear shaft.

2. In a rock drill as in claim 1, wherein the work steel has a foremost position in which it isout of pounding range of the stern and in which the hammer is engageable with the shoulder, and means is provided which is responsive to engagement of the hammer with the shoulder to disable the cup-form means from causing development of said differential pressures alternately at opposite ends of the hammer.

3. In a rock drill including a work engaging striking bar having a surface gear interrnediately of its ends, a

piston hammer reciprocable to poundthe bar against ing axially and rotationally directed thrusts developing in the gear train means between the motor and the striking bar,- wherein the gear train means includes a worm gear shaft, a worm gear carried by the worm gear shaft having a driving-connection with the gear of the striking bar, and a torsion bar drivingly connecting the rotor shaft with the worm gear shaft.

4. In a rock drill as in claim 3, wherein the rotor and worm gear shafts are hollow and arranged .in axial alignment, the torsion bar is disposed within the interior of both shafts and has a splined driven connection at one end with the rotor shaft and a splined driving connection at its opposite end with the worm gear shaft.

5. In a rock drill as in claim 4, wherein the rotor and worm gear shafts are respectively supported in a first and second pair of bearings, the motor is pneumatically operated by air carrying an oil lubricant, and passage means is provided for directing exhaust air from the motor through the first pair of the bearings to one end area of the worm gear, and other passage means is provided for directing exhaust air from the motor through a second pair of the bearings to the opposite end area of the worm gear, the torsion bar having an axially extending hole therethrough defining a connecting passage between the bearings of the said second pair.

6. In a rock drill as in claim 3, wherein the worm gear shaft has a driving spline connection with the worm gear shaft allowing relative axial movement of the worm gear, and shock absorbing means is mounted upon the worm gear shaft in abutting relation to opposite ends of the worm gear adapted to absorb axially directed thrusts imparted to the worm gear.

7. In a rock drill as in claim 3, wherein the worm gear shaft has a driving spline connection with, the worm shaft and comprises an inner thrust ring presenting a bearing surface to an end of the worm gear, an outer thrust ring presenting a bearing surface to the related bearing, and an elastomer ring between the inner and outer thrust rings. 

1. A rock drill including a casing having a piston chamber provided with a shoulder at its lower end, a floating piston hammer reciprocable in the chamber relative to the shoulder, the hammer being of cup-form provided with an axially projecting stem, a work steel having a retracted position in the casing wherein its top end is subject to being pounded by the stem as the hammer reciprocates and wherein the hammer is disabled by the work steel from engaging the shoulder, a pressure air inlet having passages for feeding pressure air to opposite ends of the piston hammer, and cup-form means reciprocable in and out of the cup of the hammer for developing pneumatic pressure differentials of such air alternately at opposite ends of the hammer to reciprocate it during the time the work steel is in its retracted position, wherein the work steel has a surface gear intermediately of its ends, a rotary motor operating independently of the reciprocating hammer has an output shaft, and gear train means drivingly couples the output shaft of the motor with the gear of the work steel; characterized in that the gear train means includes a worm gear shaft carrying a worm gear having a driving connection with the gear of the striking bar, and a torsion bar drivingly connecting the output shaft of the motor with the worm gear shaft.
 2. In a rock drill as in claim 1, wherein the work steel has a foremost position in which it is out of pounding range of the stem and in which the hammer is engageable with the shoulder, and means is provided which is responsive to engagement of the hammer with the shoulder to disable the cup-form means from causing development of said differential pressures alternately at opposite ends of the hammer.
 3. In a rock drill including a work engaging striking bar having a surface gear intermediately of its ends, a piston hammer reciprocable to pound the bar against the work, and means for alternately pressurizing opposite ends of the hammer to reciprocate the latter, a rotary motor operating independently of the reciprocating hammer having a rotor shaft, and gear train means drivingly coupling the rotor shaft with the gear of the striking bar having the capacity for absorbing axially and rotationally directed thrusts developing in the gear train means between the motor and the striking bar, wherein the gear train means includes a worm gear shaft, a worm gear carried by the worm gear shaft having a driving connection with the gear of the striking bar, and a torsion bar drivingly connecting the rotor shaft with the worm gear shaft.
 4. In a rock drill as in claim 3, wherein the rotor and worm gear shafts are hollow and arranged in axial alignment, the torsion bar is disposed within the interior of both shafts and has a splined driven connection at one end with the rotor shaft and a splined driving connection at its opposite end with the worm gear shaft.
 5. In a rock drill as in claim 4, wherein the rotor and worm gear shafts are respectively supported in a first and second pair of bearings, the motor is pneumatically operated by air carrying an oil lubricant, and passage means is provided for directing exhaust air from the motor thrOugh the first pair of the bearings to one end area of the worm gear, and other passage means is provided for directing exhaust air from the motor through a second pair of the bearings to the opposite end area of the worm gear, the torsion bar having an axially extending hole therethrough defining a connecting passage between the bearings of the said second pair.
 6. In a rock drill as in claim 3, wherein the worm gear shaft has a driving spline connection with the worm gear allowing relative axial movement of the worm gear, and shock absorbing means is mounted upon the worm gear shaft in abutting relation to opposite ends of the worm gear adapted to absorb axially directed thrusts imparted to the worm gear.
 7. In a rock drill as in claim 3, wherein the worm gear shaft has a driving spline connection with the worm gear allowing relative axial movement of the worm gear, the worm gear shaft is supported at each end in a separate bearing having a fixed position, and a separate shock absorbing unit is disposed between each bearing and a corresponding end of the worm gear.
 8. In a rock drill as in claim 7, wherein each shock absorbing unit is slidably received upon the worm gear shaft and comprises an inner thrust ring presenting a bearing surface to an end of the worm gear, an outer thrust ring presenting a bearing surface to the related bearing, and an elastomer ring between the inner and outer thrust rings. 